The present invention generally relates to gas generants used for inflating occupant safety restraints in motor vehicles.
Inflatable occupant restraint devices for motor vehicles have been under development worldwide for many years including the development of gas generating compositions for inflating such occupant restraints. Because of the strict requirements related to toxicity of the inflating gases, most, if not all, gas generants now in use are based on azides, particularly sodium azide.
However, the use of sodium azide, or other azides for that matter, results in extra expense and risk in gas generant manufacture because of the extreme toxicity of azides. In addition, the potential hazard and disposal problem of unfired inflation devices must be considered. Accordingly, nonazide gas generants provide significant advantages over azide-based gas generants with respect to these types of toxicity related concerns. Moreover, most azide-free gas generant compositions provide a higher yield of gas (moles of gas per gram of gas generant) than conventional occupant restraint gas generants.
Although an azide-free gas generating composition offers numerous advantages over an azide-based gas generant, one difficulty with the former involves reducing the production of toxic substances upon combustion to sufficiently low levels. The most difficult toxic gases to control are the various oxides of nitrogen (NO.sub.x) and carbon monoxide (CO). This problem stems from the nature of azide-free gas generants, which consist of carbon and nitrogen containing ingredients. Upon combustion, these ingredients produce small, yet undesirable levels of NO.sub.x and CO, along with the desired products of nitrogen and carbon dioxide.
In combustion processes involving compounds containing both nitrogen and carbon, it is possible to reduce or eliminate the CO or NO.sub.x by manipulating the ratio of oxidizer to fuel, but this leads to a dilemma. On one hand, increasing the ratio of oxidizer to fuel minimizes the CO, because the extra oxygen oxidizes the CO to carbon dioxide. Unfortunately, however, this approach results in increased amounts of NO.sub.x. On the other hand, if the ratio of oxidizer to fuel is lowered to eliminate excess oxygen and to provide a fuel rich condition which reduces the amount of NO.sub.x produced, then increased amounts of CO are produced.
Even though it is possible, by means of chemical equilibrium calculations, to find conditions of temperature, pressure, and gas generant composition which could reduce both NO.sub.x and CO to nontoxic levels, it has been very difficult to accomplish this result in actual practice.
This problem has heretofore been addressed in, for example, U. S. Pat. No. 5,139,588 which describes the use of additives consisting of alkali metal salts of organic acids to reduce the amount of NO.sub.x produced upon combustion of gas generants. The fuels used in these gas generants were from a group consisting of triazole, aminotetrazole, tetrazole, bitetrazole and metal salts of these compounds. These fuels are all energetic materials which result in high combustion temperatures. High combustion temperatures result in higher CO and NO.sub.x levels and although using excess oxygen to reduce CO levels and additives to reduce NO.sub.x levels improves the resulting gases, meeting existing toxicity requirements is still difficult.
One way to improve the toxicity of the combustion gases is to reduce the combustion temperature which would reduce the initial concentrations of both CO and NO.sub.x. Although simple in theory, it is difficult in practice to reduce the combustion temperature and to also retain a sufficiently high gas generant burn rate for practical automobile airbag applications. The burn rate of the gas generant is important to insure that the inflator will operate readily and properly. As a general rule, the burn rate of the gas generant decreases as the combustion temperature decreases. By using less energetic fuels, specifically fuels which produce less heat upon combustion, the combustion temperature may be reduced but the gas generant burn rate is also reduced.